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=============================================================================
FreeBSD-SA-98:04                                            Security Advisory
                                                                FreeBSD, Inc.

Topic:          security compromise via mmap

Category:       core
Module:         kernel
Announced:      1998-06-02
Affects:        FreeBSD 2.2.*, FreeBSD-stable before 1998/05/24
		and FreeBSD-current before 1998/05/19 suffer from
		this problem.
Corrected:      FreeBSD-current as of 1998/05/19
		FreeBSD-stable as of 1998/05/24
FreeBSD only:   no (also other 4.4BSD based systems may be affected)

Patches:        ftp://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-98:04/

=============================================================================
IMPORTANT MESSAGE: The FreeBSD security officer now uses the policy
ftp://freebsd.org/pub/CERT to ftp://ftp.freebsd.org/pub/FreeBSD/POLICY
for sending out advisories.
=============================================================================

I.   Background    

     The 4.4BSD VM system allows files to be "memory mapped", which
     causes the specified contents of a file to be made available
     to a process via its address space. Manipulations of that file
     can then be performed simply by manipulating memory, rather
     than using filesystem I/O calls.  This technique is used to
     simplify code, speed up access to files, and provide interprocess
     communication.

     In 4.4BSD, 4 new FFS flags were added that give the possibility
     to mark files as append-only or immutable.

II.  Problem Description

     It is possible for a process to open an append-only file
     according to the limitations of the flags, and then mmap the
     file shared with write permission even when the file is marked
     as append-only or immutable. This circumvents the concept of
     the the append-only flag.

III. Impact
     
     It is possible to change the contents of append-only files.

IV.  Workaround

     No workaround is known.

V.   Solution


     Apply one of the following patches, rebuild your kernel,
     install it and reboot your system.

     The patches below can be found on
	ftp://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-98:04/

     NOTE: Users of FreeBSD 2.2.5 or FreeBSD-current or FreeBSD-stable
     dated before 1998/03/12 will need to apply the patch mentioned in
     FreeBSD advisory SA-98:02.


     Patch for 3.0-current systems:

  Index: vm_mmap.c
  ===================================================================
  RCS file: /home/cvsup/freebsd/CVS/src/sys/vm/vm_mmap.c,v
  retrieving revision 1.75
  retrieving revision 1.77
  diff -u -r1.75 -r1.77
  --- vm_mmap.c	1998/03/12 19:36:18	1.75
  +++ vm_mmap.c	1998/05/19 07:13:21	1.77
  @@ -58,6 +58,7 @@
   #include <sys/file.h>
   #include <sys/mman.h>
   #include <sys/conf.h>
  +#include <sys/stat.h>
   #include <sys/vmmeter.h>
   
   #include <miscfs/specfs/specdev.h>
  @@ -295,12 +296,25 @@
   			 * we're at securelevel < 1, to allow the XIG X server
   			 * to continue to work.
   			 */
  -			if (((flags & MAP_SHARED) != 0 ||
  -				(vp->v_type == VCHR && disablexworkaround)) &&
  -				(fp->f_flag & FWRITE) == 0 && (prot & PROT_WRITE) != 0)
  -				return (EACCES);
  -			else
  +
  +			if ((flags & MAP_SHARED) != 0 ||
  +			    (vp->v_type == VCHR && disablexworkaround)) {
  +				if ((fp->f_flag & FWRITE) != 0) {
  +					struct vattr va;
  +					if ((error =
  +					    VOP_GETATTR(vp, &va,
  +						        p->p_ucred, p)))
  +						return (error);
  +					if ((va.va_flags &
  +					    (IMMUTABLE|APPEND)) == 0)
  +						maxprot |= VM_PROT_WRITE;
  +					else if (prot & PROT_WRITE)
  +						return (EPERM);
  +				} else if ((prot & PROT_WRITE) != 0)
  +					return (EACCES);
  +			} else
   				maxprot |= VM_PROT_WRITE;
  +
   			handle = (void *)vp;
   		}
   	}

     Patch for 2.2 systems:

  Index: vm_mmap.c
  ===================================================================
  RCS file: /home/cvsup/freebsd/CVS/src/sys/vm/vm_mmap.c,v
  retrieving revision 1.53.2.3
  retrieving revision 1.53.2.4
  diff -u -r1.53.2.3 -r1.53.2.4
  --- vm_mmap.c	1998/03/12 19:36:50	1.53.2.3
  +++ vm_mmap.c	1998/05/24 19:47:02	1.53.2.4
  @@ -57,6 +57,7 @@
   #include <sys/file.h>
   #include <sys/mman.h>
   #include <sys/conf.h>
  +#include <sys/stat.h>
   #include <sys/vmmeter.h>
   
   #include <miscfs/specfs/specdev.h>
  @@ -275,12 +276,26 @@
   			 * we're at securelevel < 1, to allow the XIG X server
   			 * to continue to work.
   			 */
  -			if (((flags & MAP_SHARED) != 0 ||
  -				(vp->v_type == VCHR && disablexworkaround)) &&
  -				(fp->f_flag & FWRITE) == 0 && (prot & PROT_WRITE) != 0)
  -				return (EACCES);
  -			else
  +
  +			if ((flags & MAP_SHARED) != 0 ||
  +			    (vp->v_type == VCHR && disablexworkaround)) {
  +				if ((fp->f_flag & FWRITE) != 0) {
  +					struct vattr va;
  +
  +					if ((error =
  +					    VOP_GETATTR(vp, &va,
  +						        p->p_ucred, p)))
  +						return (error);
  +					if ((va.va_flags &
  +					    (IMMUTABLE|APPEND)) == 0)
  +						maxprot |= VM_PROT_WRITE;
  +					else if (prot & PROT_WRITE)
  +						return (EPERM);
  +				} else if ((prot & PROT_WRITE) != 0)
  +					return (EACCES);
  +			} else
   				maxprot |= VM_PROT_WRITE;
  +
   			handle = (caddr_t) vp;
   		}
   	}

VI.   Thanks

     This advisory is based on NetBSD Security Advisory 1998-003.
     In porting the NetBSD patch, we accidentally mentioned that we
     obtained the patch from OpenBSD, which was evidently wrong.
     
=============================================================================
FreeBSD, Inc.

Web Site:                       http://www.freebsd.org/
Confidential contacts:          security-officer@freebsd.org
Security notifications:         security-notifications@freebsd.org
Security public discussion:     freebsd-security@freebsd.org
PGP Key:                ftp://ftp.freebsd.org/pub/FreeBSD/CERT/public_key.asc

Notice: Any patches in this document may not apply cleanly due to
        modifications caused by digital signature or mailer software.
        Please reference the URL listed at the top of this document
        for original copies of all patches if necessary.
=============================================================================

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